1. Field of the Invention
The present invention relates to manufacturing machine or tool elements and mechanical components that have an integrally mounted acoustic emissions sensor to produce a signal indicating operating conditions and parameters.
2. Description of the Prior Art
Manufacture of articles for consumer and industrial use may employ metal removal operations (such as drilling, milling, turning, and grinding), metal forming and primary processes (sheet rolling, sheet forming, drawing, and ironing, in addition to forging and cold forming), as well as joining processes (such as welding). In all of these operations, plastic deformation is invariably involved. It occurs in welding due to solidification shrinkage (small strains). In metal forming and other primary processes, larger shape changes occur which vary with the nature of the specific process (small, medium, and large strains). In metal removal operations, plastic strains imposed can be quite large. Grinding is an example.
Plastic deformation in all crystalline materials involves defect processes traceable ultimately to the "line defects," i.e., crystal dislocations. Shape change or permanent strain is enforced by causing large scale dislocation activity, involving millions of dislocations per cm.sup.3 or inch.sup.3.
In high strain-rate (high speed) processes, dislocation-lattice interactions involve phonons. Substantial acoustic emission can occur, as, for example, the loud noise accompanying metal fracture. At slower strain-rates, a rich variety of dislocation interactions can occur, accompanied by elastic wave emission (acoustic emission). Many of these phenomena are well discussed in recent publications of workers at the National Bureau of Standards. Among the best known examples of acoustic emission within the audible range is the "cry of tin" caused by twining of individual crystals within a block of tin.
Ceramic and ceramic-like materials (graphite in a lead pencil is an example) also produce substantial acoustic emission as a consequence of elastic energy release accompanying fracture. Large scale analog of this phenomenon are the seismic waves accompanying earthquakes. In this case, the displacements accompanying seismic wave propagation are large enough to be detected by even crude seismometers.
Similar surface displacements accompany acoustic emission in metals and ceramic objects. As noted earlier, they are traceable to dislocation-dependent microscopic processes. The displacements produced are small.
Mechanical components subjected to repeated loading or cyclic stresses eventually fail by metal fatigue. Substantial dislocation activity is known to accompany fatigue damage accumulation. When the accumulating damage processes can no longer be accommodated by internal microscopic changes and atomistic displacement, failure initiation occurs. Subsurface and surface cracking in the microscopic scale, surface pitting and other similar damaging events follow. Each of these failures inducing events is accompanied by acoustic emission.
Once a microscopic or observable crack is formed, continued cyclic stressing or repeated stressing induces bursts of acoustic emission events. Discrete crack growth events and the accompanying elastic energy release are responsible for burst acoustic energy emission.
Acoustic emission sensors have been advanced for mounting on machine tool and mechanical parts for sensing acoustic emissions. Conventionally, these sensors with inertial masses are mounted on a support for the tool or machine element being monitored. This results in mechanical filtering of acoustic emission signals between the mounting interfaces of the tool. Because the acoustic emission signals are relatively low level, and in a relatively high frequency range, the filtering results in the inability to accurately follow the pattern of acoustic emissions from a given tool or machine element. Commercially available sensors are coupled to the objects, generally by pressing the sensor against the surface of the object. Use of rubber bands is common, and sometimes adhesives are used to hold the sensor in place. Of course, any imperfection in the interface between the sensor and the object on which it is mounted also acts as a filter, and thus various coupling agents, such as liquids, are interposed between the sensor and the object. Despite this, transduction efficiency is low, and at the present time the manufacturers of existing acoustic emission detection systems recommend low noise, very high gain amplifier systems. The problems associated with the high gain amplifiers of course include any background noise, and rather complex circuitry for obtaining any type of a usable signal.
The use of piezoelectric material for sensing acoustic emissions is known, but these generally are mounted onto a sensor assembly having an inertial mass. The sensor assembly is mounted onto a support on the manufacturing tool or in the location where acoustic emission sensing is desired. A study of these types of devices is set forth in Ferro Electrics, 1981, Volume 32, pages 79-83, in the article entitled "Durable Lead Attachment Techniques for PVDF Polymer Transducers With Application to High Voltage Pulsed Ultrasonics," by Scott et al.
In particular, page 82 of the Scott article shows a response of two different types of sensors including a commercial broad band acoustic emission transducer, and the PVDF sensor under consideration.
Thus, it has been recognized that materials have atomic and intermolecular structures that are subject to shear, and void and discontinuity producing events. When such events occur, they release elastic strain energy in the form of stress waves. These waves propogate through the solid in the forms of acoustic waves, and velocity is determined by the structure and properties of the solid. The acoustic waves may possess frequencies up to several Mhz and are eventually dissipated by transmission reflection and refraction at the boundary surfaces of the solid and by irreversible processes within the solid, such as molecular shifts. Monitoring of these sound wave or acoustic wave changes also gives information about friction conditions between moving surfaces, and similar acoustic wave producing events.
In the prior art, lead zirconate titanate (PZT) detectors with inertial masses are used and they provide for substantial "ringing" at the ends of the signals being received as shown in the Scott article cited above. Additionally, in that article the PVDF (polyvinyldene flouride) piezoelectric polymers were shown to produce a transducer without substantial ringing. Both of these materials are piezoelectric and they can be used for the present integrated acoustic emission sensor that provides real time analyzation of acoustic emissions of a machining tool element.